I don't think that was explained in-game, but I always got the impression that Bevelle was the aggressor since they completely outmatched Zanarkand. The Bahamut Fayth tells Tidus this:Originally Posted by Queen Anthai
Fayth
"A war between Zanarkand and Bevelle."
"Bevelle's machina assured their victory from the start. Spira had never seen such power."
"The summoners of Zanarkand didn't stand a chance."
"Zanarkand was doomed to oblivion."
"That's why we tried to save it--if only in a memory."
You're welcome, by the way. I was glad to help.